Load-bearing, lightweight, and compact super-insulation system

ABSTRACT

A lightweight and compact super-insulation system that is also capable of supporting a high level of compressive load is described. The system utilizes spacers to provide structural support and utilize controlled buckling of a thin protective outer skin supported by spacers to form strong catenary surfaces to protect insulation material underneath. The spacers may comprise an aerogel, or an aerogel may provide insulation separate from the spacer yet contained within the thin outer skin. The system will be useful for thermal management of variety of deep underwater structures such as pipe-in-pipe apparatus, risers or subsea trees for ultra-deep water oil-and-gas exploration.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/468,365, filed May 6, 2003, the entire teachings of which areincorporated herein by reference.

BACKGROUND

In deep- and ultra-deep-water oil and gas exploration, crude oil or gasis extracted from below the sea floor via a pipeline system to the watersurface. It is important to maintain the temperature of the hot crudeoil or gas flowing in the pipe above about 30-50° C. depending on thecomposition of the hydrocarbons (e.g., crude oil or natural gas).Maintaining a temperature in this range prevents flow restrictions orclogging due to formation of hydrates or wax, which can occur viacooling of the crude oil or gas by cold water as the hydrocarbons flowfrom the underwater well to the production plant on the surface. Also,if the well must be capped for maintenance or due to inclement weather,it is highly desired to keep the temperature of the hydrocarbon insidethe pipe and other parts of the pipeline systems (e.g., a Christmas treeor subsea tree, risers, etc.) above precipitation temperature for aslong as possible to minimize or avoid expensive and time-consumingde-clogging processes before resuming the pumping operation.

These are the so-called flow assurance requirements for the underwaterpipe-in-pipe configuration. The pipe-in-pipe configuration has been thetraditional method of choice to satisfy the flow assurance requirementsof the deep-water exploration. The configuration uses two thickconcentric pipes; i.e., the inner pipe (flow line or flow pipe) and theouter pipe (carrier pipe). The flow line carries the hydrocarbon comingout of the well at high temperature (e.g., 60-300° C.) and at highpressure [e.g., up to about 70 MPa (10,000 psi)]. The carrier pipe isdesigned (independent of the flow line) to withstand the externalhydrostatic pressure that proportionately increases with depth [e.g.,about 28 MPa (4000 psi) at 2800 m].

In the annulus between the two pipes, split-ring spacers (also referredto as “centralizers”), which are made of a material having relativelylow conductivity (e.g., a polyamide), are installed at regular intervals(e.g., at 1.2-m intervals). The spacers act as a guide during theinsertion of the inner pipe into the outer pipe; each pipe can be 1 or 2km in length. The spacers are also designed to help maintain the annulargap between the two concentric pipes when the pipe-in-pipe apparatus isbent for winding onto a spool or when it bends after installation. Inthe annular gap between the spacers, insulation material is wrappedaround the flow line. The insulation material can be, e.g., a urethanefoam having a thermal conductivity of 24 mW/m-K and higher or a fumedsilica product packaged in vacuum with thermal conductivity of 21mW/m-K.

In some locales, the temperature of the crude oil coming out of the wellis only 60° C., which is not very hot, as is the case off the coast ofAngola. As a consequence of this relatively low temperature, a muchhigher level of insulation is needed to prevent hydrate formation due tocool down. Also, as the recoverable oil and gas deposits in the shallowsea bottom are exhausted, the wells will be drilled in increasinglydeeper waters. The current pipe-in-pipe design, while acceptable down toa depth of 1000 m, meets severe obstacles when the underwater field getsmuch deeper than 1000 m, as described below.

As the well depth increases, the following obstacles and technicalissues have to be overcome. As a starting point, the characteristics ofhydrocarbons become more prone to forming wax or hydrates. Additionally,since the distances between the deeper wells and the production plant onthe surface platform are significantly increased, theoverall-heat-transfer (OHT) value of the pipe-in-pipe apparatus mustordinarily be reduced to very low values, such as 0.5 W/m²-° C. with atransient cooling requirement of less than 30° C. in 16 hours, toprevent over-cooling of the recovered hydrocarbons. Providing apipe-in-pipe apparatus with this very-low OHT value would ordinarilynecessitate significantly increasing the thickness of insulation, whichin turn would increase the inner diameter of the carrier pipe needed toaccommodate the additional insulation contained within the carrier pipe.

As the inner diameter of the carrier pipe increases, the carrier-pipewall thickness that is needed to withstand a fixed external pressure inthis context increases as an approximately proportional function of theincrease in the outer diameter of the carrier pipe. Moreover, as thedepth increases, the external pressure acting upon the carrier pipeincreases as a linear function of the depth. For each 10.33 m of waterdepth, pressure increases by 1 atm (100 kPa). At 2500 m, the hydrostaticpressure reaches about 25 Mpa (3560 psi). The thickness of the carrierpipe wall is increased approximately proportionally with an increase inthe hydrostatic pressure for a given inner radius. Therefore, thecarrier pipe wall is fabricated with increasing thickness as thepressure for the intended usage is increased, which causes furtherincrease in the outer diameter of the carrier pipe as the intended usagedepth increases.

As the carrier pipe gets larger in diameter and in thickness, thefollowing disadvantages result. First, the weight of the pipe-in-pipeapparatus increases sharply, increasing approximately proportionallywith the square of the wall thickness and linearly with the meandiameter. Second, the material cost increases as the amount of steel andinsulation increases. Third, additional labor and heavier equipment isneeded to produce the pipe-in-pipe apparatus. Fourth, heavier equipmentis needed to wind the pipe-in-pipe apparatus onto a spool and also toinstall the pipe-in-pipe apparatus; the equipment that is currently usedmay need to be reinforced and strengthened (at a significant level ofcapital expenditure) to handle the much heavier pipes that would berequired for depths of 2500 m and deeper. Fifth, the submerged weight ofthe pipes can become too heavy for the currently used derricks or shipsto handle the load and keep it stable in rough seas; the excess weightof the pipes accordingly necessitates building larger derricks, shipsand larger buoyancy tanks at increased costs and decreased stability inthe rough waters. Finally, the ship must make more trips to transportthe necessary pipe lengths.

The current pipe-in-pipe manufacturing operation is extremely laborintensive and therefore costly. The pipes used for flow lines andcarrier pipes generally come in 12-m (40-ft) lengths from the supplier.At the factory, the 12-m long pipes are first welded together into 1- or2-km long sections of the outer carrier pipe. Section by section,polyamide spacers (centralizers) are installed onto the inner flow-linesections and thermal insulation is wrapped around the flow-line sectionsbetween the centralizers. After each flow-line section is insulated andsecured by strapping with tapes, that section will be pushed into awaiting carrier pipe. The next section of the flow line is welded to thesection to be inserted into the carrier pipe. The centralizers helpguide the flow line during insertion into the carrier pipe. This processcontinues until the full length of the 1-km or 2-km pipe-in-pipeapparatus is assembled. The processes of welding the pipes andinstalling the centralizer and thermal insulation occur instop-and-start fashion and require substantial manual labor and time.

In an alternative method currently used, the entire 1- or 2-km sectionsof the flow line and the carrier pipe are welded separately. Then, theentire length of the flow line is fitted with centralizer rings atregular intervals and with thermal insulation between the centralizers,and covered and fastened in place with tapes. The completed insulatedflow line is then carefully inserted into the waiting carrier piperelying on the centralizers to maintain the annular gap and thus protectthe insulation during the insertion operation.

For the reasons discussed above, when the depth increases significantlyfor the underwater pipeline, it will become more economically andlogistically unacceptable to continue to use the current design ofpipe-in-pipe apparatus, insulation material, and manufacturing process.Both manufacturing methods described above represent the state-of-theart pipe-in-pipe manufacturing process and are very labor intensive,costly and slow.

In one recent pipe-in-pipe design, the inner flow line is covered withnon-load-bearing insulation protected by a carrier pipe made of GlassReinforced Plastic (GRP). The GRP pipe is connected mechanically to theflow line at both ends of a 12-m long pipe section using mechanicaljoints comprising a load-bearing polymeric material able to guaranteethe thermal and mechanical performances. A relatively high-performance,but non-load-bearing insulating material with thermal conductivity ofapproximately 21 mW/m-K fills the annular space between the flow lineand carrier pipe and provides the required thermal performances. Theauthors describe how much lighter this new pipe would be compared to thecurrent pipe-in-pipe design and how much more conveniently the pipecould be produced in an automated process. Although the submerged weightof this new configuration may be less than that of a conventionalpipe-in-pipe apparatus designed for the same operating conditions, theability of the fiberglass to withstand the seawater conditions on along-term basis is unproven, and the outer layer remains extremelythick. Therefore, the GRP pipe may not have the necessary bendingflexibility, and, as a consequence, additional trips may be needed tocarry the larger diameter pipes to installation sites.

In the conventional pipe-in-pipe design and also in the GRP pipe-in-pipedesign, described above, the inner pipe is designed to withstand theusually high inner pressure [e.g., 70 Mpa (10,000 psi)], and the outerpipe is designed to independently withstand the external crushingpressure [15 Mpa (2170 psi) at 1.5 km (5000 ft) and 30 Mpa (4340 psi) at3 km (10,000 ft)].

SUMMARY

The thermal insulation systems of this disclosure can be used for suchdiverse applications as deep-water pipeline insulation, LNG tankerinsulation, process piping, etc. These systems insulation systems can becharacterized as follows: lightweight, thin, low-cost, highthermal-insulation performance and high load-bearing capability, as wellas being easily installed and maintained. Pre-existing insulationsystems may satisfy some of the above desired properties but not all ofthem. For example, evacuated multiple layer insulation (MLI) encased inheavy metal frames performs marvelously in terms of load-bearingcapability and thermal performance; but, in general, MLI's are extremelyheavy and expensive and are very difficult to manufacture, install andmaintain. On the other hand, low-density silica aerogels offer excellentinsulation with up to five times the thermal-insulation performance ofcommonly used fiberglass in ambient conditions, though the low-densityaerogels typically cannot support the load beyond a fraction of anatmosphere before getting compressed. Fiberglass is cheap, but it is toobulky and ineffective; moreover, fiberglass is non-load-bearing, and itsinstallation is messy. Foam can be load-bearing to a very limitedextent, and the thermal-insulation performance is too low.

The advanced embodiments of insulated structures, described below, canbe used for deep-water and especially for ultra-deep-water oil-and-gasexploration and other applications. The structure includes a thinprotective outer skin and an underlying structure contained by the outerskin. One or more spacers are provided between the outer skin and theunderlying structure. The spacers provide structural support for theouter skin and can enable the outer skin to be deformed to producecatenaries between spacer contact surfaces to place the outer skin undera tensile stress when subjected to an external pressure load. In oneembodiment, the insulated structure is a pipe-in-pipe apparatus, whereinthe outer skin is a carrier pipe with thin walls and the underlyingstructure is a flow line for transporting hydrocarbons.

When designed for a given set of operating conditions, the new designcan offer the following advantages and salient features over thestate-of-the-art: (a) a much thinner (by nearly an order of magnitude)wall of the outer skin; (b) a much smaller carrier-pipe outer diameter;(c) a drastically reduced total weight (reduced by close to one halfthat of the conventional design); (d) a higher flexibility and a tighterbending radius (approaching that of the inner pipe alone in thestate-of-the art or GRP pipe-in-pipe design); (e) more effective thermalinsulation between the underlying structure and the outer skin; (f) alower material and fabrication cost; (g) a smaller spool diameter orfewer spools for the same length of a pipe-in-pipe installation; and (h)a lower installation and maintenance cost, etc. Additionally,pipe-in-pipe designs of this disclosure are eminently suitable forautomated fabrication in mass scale to drastically reduce the labor costand therefore reduce the total cost of the pipe-in-pipe apparatus.Further still, fewer trips to the installation site need be made tocarry the same length of pipe-in-pipe apparatus for spool or J-layinstallation.

Even though various principles of this invention can be applied to manyparts of the subsea system, the description of the invention providedhere will focus, for the sake of simplicity of presentation, on apipe-in-pipe application.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings, described below, like reference charactersrefer to the same or similar parts throughout the different views. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating particular principles of the methods and apparatuscharacterized in the Detailed Description.

FIG. 1 is an exposed, perspective view showing a flow line, a helicalspacer and a carrier pipe.

FIG. 2 is a magnified view of the apparatus of FIG. 1.

FIG. 3 is a partially cut-away perspective view of the detail of aspacer having a triangular cross-section, with a weld strip andload-bearing insulation.

FIG. 4 is a perspective view of a thin-walled carrier pipe in catenarydeformation under external load to rely on the high tensile strengthinstead of the low collapse strength of the carrier pipe.

FIG. 5 is a preliminary stress plot of an advanced pipe-in-pipeapparatus with a helically wound spacer having a triangular crosssection.

FIG. 6 illustrates a heat pipe system that uses geothermal energy tomaintain the warmth of hydrocarbons in a pipe-in-pipe apparatus.

FIG. 7 is a close-up view of various layers of the heat pipe system andpipe-in-pipe apparatus of FIG. 6.

DETAILED DESCRIPTION

Traditional pipe-in-pipe designs, and even the recent GRP design, relyon the outer carrier pipe having sufficient thickness and strength tohandle the external load on its own without collapsing under normaloperating conditions. By necessity, this makes the outer wall of thecarrier pipe relatively thick.

When a pipe is exposed to an external pressure, q, the failure modeordinarily is that of pipe collapse due to elastic buckling instabilityunder compressive load, which is approximately given by: $\begin{matrix}{q = {\frac{1}{4}*\frac{E}{1 - v^{2}}*\frac{t^{3}}{r^{3}}}} & \left( {{Eqn}.\quad 1} \right)\end{matrix}$(Warren C. Young, Formulas for Elastic Stability of Plates and Shells,Roark's Formulas for Stress and Strain, Sixth Edition, McGraw-Hill,Inc., Equation 19.a, Table 35, 1989), where q is the external pressure;E is Young's Modulus; v is Poisson's Ratio; t is the wall thickness; andr is the mean radius. For a long tube held circular at intervals of l,$\begin{matrix}{q = {0.807*\frac{E}{l}*\frac{t^{2}}{r}\sqrt[4]{\left( \frac{1}{1 - v^{2}} \right)^{3}*\frac{t^{2}}{r^{2}}}}} & \left( {{Eqn}.\quad 2} \right)\end{matrix}$(Warren C. Young, Formulas for Elastic Stability of Plates and Shells,Roark's Formulas for Stress and Strain, Sixth Edition, McGraw-Hill,Inc., Equation 19.b, Table 35, 1989).

As an example, let us take a pipe with the mean radius of 6 inches (15cm) made of steel with a tensile strength of 80,000 psi (550 MPa), aYoung's Modulus of 30 million psi (200 GPa), and a Poisson's Ratio (v)of 0.29, which is subjected to 4,000 psi (28 MPa) of external pressure.Since the pressure is external, the failure mode is that of bucklinginstability; and the necessary wall thickness to resist collapsing dueto buckling instability, calculated from Equation 1, is 0.47 inches (1.2cm). When there is an internal support, such as spacers, however, thesituation markedly changes for the better. For instance, if the spaceris thin and circular and is placed regularly apart along the length ofthe pipe such that l=6 inches (15 cm), the required wall thickness,calculated from Equation 2, is only 0.18 inches (4.6 mm), whichrepresents a 62% reduction in wall thickness.

As illustrated in FIG. 1, a relatively thin-walled carrier pipe 14 canbe well supported by the spacers 10 (use of the term “spacers” hereincan refer to discrete spacers or turns of a helically wound continuousspacer) in the annulus, where the spacers 10 are firmly supported by theflow line 12. In a first embodiment, the thin-walled carrier pipe 14 ismechanically supported, in the case of using insulation material thatcannot bear the external pressure load, by strategically placed spacers10 in the annulus of the pipe-in-pipe apparatus along the length of thepipes 12, 14. The carrier pipe 14 forms a catenary surface between thespacers 10, and the principal stress experienced by the thin catenaryforming wall of the carrier pipe 14 under load is tensile. As usedherein, references to a “catenary” shape or “catenary-like” shape referto shapes that fairly closely match the graph of a hyperbolic cosinethat characterizes a perfect catenary. However, a perfect catenary isonly possible with perfectly flexible materials, which fits few if any“real world” structures. Accordingly, references to catenary orcatenary-like shapes herein includes those shapes that vary somewhatfrom perfect catenaries due to limitations in flexibility and other“real world” influences, interferences and limitations.

Placement of the spacers 10 is a matter of optimization. If the spacingis too large, the wall thickness of the carrier pipe 14 will have to bethick; and if the spacing is too small, the wall thickness of thecarrier pipe 14 will be thin but the thermal conduction loss through thespacer 10 will increase. For optimization of spacer 10 spacing, thematerial properties of the spacer 10, such as thermal conductivity andmechanical strength of the carrier wall, are taken into account toarrive at the right spacing. Greater mechanical strength of the carrierwall and greater thermal conductivity of the spacer 10 both encouragethe adoption of greater distances between the spacers 10 (or betweenrevolutions of a spacer in a helical configuration). The spacers 10 canbe made from structurally strong material such as steel or high-strengthcomposites. In particular embodiments, the spacer 10 includes a separatelayer of thermal insulation that can withstand the compressive load.

In a second embodiment, the thin-walled carrier pipe 14 is mechanicallysupported by an advanced insulation in the annulus (that has bothexcellent thermal-insulation capability and excellent compressivestrength). The thermal conductivity of the insulation can be, e.g., 50mW/m*K or less. The principal stress experienced by the thin wall of thecarrier pipe 14 under external pressure load in this embodiment isradial and mostly compressive. The advanced insulation in this secondembodiment has sufficient structural strength to withstand themechanical load and can provide an excellent level of thermalinsulation. It is also a special case of the first embodiment where thespacer is made of the special structural insulation material and fillsthe annulus along the length of the pipes rather than being placed apartat intervals as in the first embodiment.

For example, the spacer can be formed of an aerogel, such as apre-conditioned silica aerogel or a high-strength cellulose aerogel,such as those available from Aspen Aerogels (Northborough, Mass., USA).Aerogels are described in greater detail in U.S. Pat. No. 6,670,402,which is incorporated herein by reference in its entirety. A silicaaerogel can be pre-compressed to the maximum pressure level anticipatedin the operation; precompressed silica aerogels have been found to showlittle deterioration in thermal performance for the same thickness afterthey are compressed. Cellulose aerogels exhibit extremely highstructural strength even without precompression, while still providingexcellent thermal insulation performance.

In the design illustrated in FIGS. 1-3, a thin spacer 10 is helicallywound over the flow line 12. The angle of the helix is between zerodegrees (i.e., discrete spacer rings) and eighty degrees, where theactual angle depends on the width of the spacer used and the requiredgap between spacers. The spacer 10 supports a relatively thin carrierpipe 14 (see FIG. 2). In the embodiment of FIG. 3, the triangular crosssection of the spacer 10 is evident; the triangular cross section isespecially amenable to a winding operation and is capable of handlingwell the concentrated load coming from the carrier pipe 14 underexternal load. A weld strip 16 (optional) and load-bearing insulation 18(optional) can also be seen in FIG. 3. The optional weld strip 16 iswelded to the spacer 10 (extending beyond the flat top surface of thespacer 10) and to the carrier pipe 14 and serves to ensure the integrityof the weldment and to provide high weldment strength. The weld strip 16also spreads the external load, thereby reducing the stressconcentration in the carrier pipe 14 under operating load. The optionalload-bearing insulation 18 will ensure significant reduction in the heattransmitted from the wall of the carrier pipe 14 through the weld strip16 and through the main structural body of the spacer 10 with thetriangular cross-section. The spacer 10 envisioned here structurallysupports the carrier pipe 14 and also thermally separates the flow line12 and the carrier pipe 14 very effectively.

The wall of the carrier pipe 14 is relatively thin and is allowed, bydesign, to collapse under normal operation to form a catenary betweenspacers strategically located underneath the carrier pipe 14, as shownin FIG. 4. In this case, the initial collapse is immediately turned intoa tensile loading of the carrier pipe 14. Accordingly, failure of thecarrier pipe 14 will require overcoming the high tensile strength of thecarrier pipe 14 instead of being a function of the relatively lowcollapse strength of the carrier pipe 14.

It was already illustrated using Equations 1 and 2 that the catenarydesign results in a 62% reduction in the thickness of the carrier pipeif the spacer rings (e.g., consecutive rotations of the spacer in ahelical configuration, measured at a common radial position) are spaced15 cm (6 inches) apart for the same flow-pipe diameter (i.e., 10 cm) andoperating conditions given previously. This reduction in thickness willsignificantly decrease the weight of the carrier pipe 14. Of course, interms of keeping account of the total weight of the pipe-in-pipeapparatus, one has to add the weight of the spacers 10. However, theweight contribution of spacers 10 averaged over the entire span would berelatively small and would be far outweighed by the weight savingsresulting from use of the thinner carrier pipe.

The spacers 10 are supported in the radial direction by the flow linepipe 12. The spacers 10 take the load transmitted through the contactsurface with the carrier pipe 14. Under pressure loading both frominside the flow line 12 and from outside the carrier pipe 14, thespacers 10 become the mechanical link between the carrier pipe 14 andthe flow line 12. In fact, this bi-directional load transmission has thebeneficial effect of at least partially balancing the inner tubepressure of the hydrocarbons with the outer pressure of the sea water.For example, if the inner pressure coming from the hydrocarbon is 69 MPa(10,000 psi) for the flow line pipe 12, and if the outside pressure forthe carrier pipe 14 caused by the sea water is 28 MPa (4000 psi), theeffective loading on the flow line 12 is only 41 MPa (6000 psi). Seenanother way, the spacers 10 can be regarded as “girders” for the flowline 12. Because of the pressure balancing and the girder effects of thespacers, the wall thickness for the flow line 12 can also be reduced, inaccordance with the effective reduction in the loading experienced bythe flow line 12 as described above.

Spacer Design

The spacers can be discrete (e.g., in the form of separate rings) asimplied by Equation 2. Alternatively, the spacers can be of continuoushelical design as shown in FIG. 1. In a limiting case, a spacer willturn into a full cylinder occupying the annulus formed between the innerflow line and the outer carrier pipe. Apart from the above-mentionedlimiting case, the cross section of the spacer can be either solid ortubular (hollow) and can take a variety of shapes, such as a circle ortriangle, depending on the material and the operating conditions onechooses. FIGS. 1-3 show a rendering of a helically wound spacer with aspecial cross section designed to perform multiple functions well. Thecross-section of the spacer should be such that it will be conducive tobeing bent around the pipe without collapsing the tubes. For example,triangular, circular, elliptical and trapezoidal tubes can be easilywound around a pipe in a controlled manner without overly collapsing theinterior volume of the tube.

FIG. 3 shows the details of this particular helical spacer 10 thatincludes a tubing of triangular cross section between a load-bearing andinsulating strip 18 at the bottom and a flat strip 16 on top. Thetriangular shape was chosen to handle the compressive loading and tominimize the heat transfer from the carrier pipe 14 to the spacer 10 andto the flow line 12. The overall heat transfer value from the carrierpipe to the flow line can be, e.g., 5 W/m²-° C. or less. The lower part(i.e., the side facing the flow line pipe 12) of the spacer 10 has aload-bearing and insulating strip 18 in the form of ahigh-compressive-strength aerogel strip designed to thermally isolatethe metal spacer coil 10 from the inner flow line 12. The aerogel stripcan be formed of pre-compressed fiber-reinforced silica aerogels orcellulose aerogels, both having the requisite properties of highstructural strength and excellent thermal insulation. The thickness ofthe strip will be determined by the insulation requirement for thespacer 10 and will generally range between 1 mm to the full gapdimension of the annulus between the flow line 12 and the carrier pipe14, the latter case signifying the use of structural insulation as thefull spacer 10. The flow line 12 is designed to handle the high-pressurehydrocarbon flow just like the flow lines currently in use.

FIG. 5 shows a result of a preliminary finite element analysis thatconfirms the viability of the pipe-in-pipe apparatus with aspacer-supported thin outer skin. The spacer in FIG. 5 is helicallywound and has a triangular cross section. The plots of FIG. 5 illustratethat the stresses in the outer wall are relatively low directly abovethe spacers and higher in the catenaries between the spacers.

The spacers 10 are also configured to provide no more than minimal heattransfer between the outer carrier pipe 14 and the inner flow line 12while providing the necessary mechanical support for the thin outer skin(i.e., carrier pipe 14) that forms a catenary between spacers 10 in thecase where there is an axial gap between spacers 10. The spacers 10,regardless of their design, are chosen to give very-high thermalresistance either via small contact areas preferably at the interfacesbetween the spacer 10 and the carrier pipe 14 and the flow line 12, asshown, or by using a load-bearing thermal insulation material for thespacers 10 or by placing a strip made of such a material between thespacers 10 and one or both of the pipes 12, 14. The spacers 10 can bediscrete ring spacers or can be helically wound strips or tubes ofvarious cross-sections.

The spacer cross-sections include, but are not limited to, tubes ofcircular or triangular cross-sections or solid rods of rectangular,circular or triangular cross-sections. The tubular spacers can be eitherevacuated, pressurized with fluids, or in pressure equilibrium with theannular space through breather holes.

Insulation Materials

The gap created between the two concentric pipes and the spacers can beevacuated, evacuted with radiation shields, partially or entirely filledwith insulation materials, or simply filled with gases. Preferably,insulation materials such as low thermal conductivity gases, aerogels,or any other effective insulation material can be inserted in theannular space created between the flow line 12, the spacer 10 and thecarrier pipe 14 depending on the requirements for the pipe-in-pipeinstallation and application at hand.

Pipe-in-Pipe Manufacturing Process

The new pipe-in-pipe apparatus can be produced in factories, on shore,or if necessary on board the ship. The machinery components can be linedup or arranged along the length of the pipe being manufactured toperform the manufacturing operation according to the following sequence:

-   -   1. At a pipe handling section, lengths of delivered pipe        (usually in 12 m lengths) are loaded and fed for use as the flow        line 12.    -   2. At a flow-line welding station, the individual lengths of        pipe are welded to eventually form continuous sections of the        flow line 12, usually in 1 or 2 km lengths.    -   3. A pipe feeder/rotator assembly forms the core of the entire        fabrication operation by providing the linear and rotational        motion of the ever-lengthening pipeline that will form the flow        line 12. The assembly includes a linear thruster that pushes the        pipe forward. The linear thruster, itself, is mounted on a        rotator that rotates the long pipe. Combined, the assembly        provides the linear as well as the rotational motion for the        pipe as it undergoes various additions to form the advanced        pipe-in-pipe apparatus. The helical angle and the pitch of the        spacer 10 are controlled by the relative speeds of linear motion        and the rotational motion provided by the feeder/rotator        assembly.    -   4. The stock for the spacer 10 (e.g., a triangular tube        sandwiched between a flat aerogel strip and a welder strip) is        linearly fed at a spacer station and wound up onto the inner        pipe in a helical fashion at a desired pitch and helical angle        by the linear/rotating motion of the flow line 12.    -   5. The stock for the insulation is linearly fed into the empty        volume between the helical turns of the spacer 10 at an        insulation station and wound up onto the flow line 12 at a        desired pitch and helical angle by the linear/rotating motion of        the flow line 12. Consequently, the insulation will fill the        space between the helically wound spacers 10. Multiple layers of        the insulation can be provided either at separate substations or        all at once at this station. After the proper amount of        insulation is wound onto the flow line 12, the insulation is        secured by a fastening layer at this station.    -   6. At an outer skin welding station, a thin metal strip that        spans the gap between the helically wound spacers 10 is fed and        welded onto the flat top portion of the spacer. The weldment        consists of two adjacent metal strips that form the outer skin        (i.e., carrier pipe 14) and the flat strip 16 centered along the        top weld strip of the spacer 10 that forms the underlayment for        the welding. This configuration ensures that the weldment of the        carrier pipe 14 at the seams will be more secure and that the        stress on the thin outer skin under design external pressure        will be within an acceptable limit.    -   7. At a cleanup section, the weldment for the outer skin is        cleaned in preparation for the coating operation.    -   8. At an end treatment section, the proper end sections, with        provisions for pulling the finished pipeline, are attached both        at the beginning of the long (e.g, 1 or 2 km) pipe section and        at the end when the desired length of the flow line section is        completed.    -   9. At a painting/coating section, the finishing touches are put        on the advanced pipe-in-pipe apparatus by applying coating        layers for various purposes, such as corrosion protection, rust        prevention, etc.    -   10. At the finished section transport section, the final stages        of the manufacturing process are performed. Once a 1 km (or 2        km) section is finished, the finished section will be rolled        into a storage rack to be connected into much longer sections        such as 10 or 20 km lengths onto a spool or to form a long        floating pipeline to be towed to an installation area.

As is evident in the description given above, the entire operation canbe automated with minimal involvement by human operators. The machinecomponents described above are not particularly high tech or high cost.Consequently, the advanced pipe-in-pipe design is eminently amenable tolow cost handling and manufacturing in contrast with the currentpractice of using a lot of manual labor and human involvement.

Use of the Load-Bearing, Lightwright and Compact Superinsulation System

So far, the new load-bearing, lightweight, compact, super-insulationsystem has been described for deep and ultra-deep underwater structureapplications among others. The system can be applied to many parts ofthe underwater oil exploration system that requires effective thermalinsulation under high external pressure, such as a flow line, a riser, aChristmas tree or subsea tree, in-field lines, and any other parts thatwould benefit from compact, lightweight super insulation with arelatively thin protective skin. Similar systems can be easily extendedto insulate the LNG tankers and any other applications where the highload-bearing capability is required.

Special Embodiments:

Insulation for a Subsea Tree:

A Christmas Tree (subsea tree) of an underwater pipeline installationhas many surfaces that are not tubular; for example, the surface thathas to be insulated under pressure loading can be largely flat, flat,curved, or irregular. In such cases, a variation of the insulationsystem described thus far for pipe in pipe can be used; namely, stripsof spacers can be attached to the surface of an inner conduit withappropriate spacing and, just like in the case of the pipe-in-pipeapparatus, insulation material can be installed between spacers. Theinsulation and the spacer grid will then be covered with relatively thinouter-skin sheet. Here, the idea is again to have the thin outer skin towithstand the external pressure loading by forming catenary surfacessupported by the spacer grid underneath, as described in the case of thepipe-in-pipe apparatus. Of course, the spacers will be designed andplaced with sufficiently narrow spacing and sufficient height to preventexcessive compression of the underlying insulation and to distribute thecompressive load across the surface of the underlying structure.

Insulation for a Liquefied Natural Gas (LNG) Tanker

A brief explanation as to how this insulation system would be used toeffectively insulate a large system, such as an LNG tanker, follows.Unlike a pipe-in-pipe apparatus, the LNG tanker carries a large volumeof liquefied natural gas inside the tank. The tank undergoes significantgeometric/dimensional changes when the liquefied natural gas, which isat a cryogenic temperature, is introduced. The insulation system isdesigned either to move with the tank when it shrinks and expands or tobe localized in order to avoid large displacements if all the spacersare connected throughout. In either case, there will be relative motionbetween the insulation system and the tanker or the outer shell if it isa double shell design depending on where the insulation system isphysically attached.

For the sake of simplicity, we will assume that the insulation system isattached to a flat outer shell and the flat-bottomed LNG tank issupported by the insulation system. The relative movement of the LNGtank would be in the x-y plane at the interface between the LNG tank andthe bottom plate. In this case, the insulation system includes (a)spacers that bear the weight load of the vessel containing the liquefiednatural gas and (b) thermal insulation, such as a non-load-bearingaerogel, placed between the spacers. The spacers include load-bearinginsulating (e.g., aerogel) strips to minimize heat conduction throughthe spacers. It is also possible to use load-bearing aerogels to fillthe gap between the LNG tank and the outer wall and, in a special case,without the use of separate spacers. In the special case, theload-bearing aerogel layer is the spacer.

Insulation and Support Structure for a Flow Pipe for TransportingLiquified Natural Gas:

In another embodiment, the inner flow pipe carries liquified natural gasat ambient pressures or at slightly elevated pressures and is furthermechanically supported by spacers (a.k.a., centralizers) or othermechanical structures that are placed in the annular space between thecarrier pipe and the flow pipe. Insulating materials, such as aerogelparticles or aerogel blankets, are positioned in the annular space toeffectively insulate the fluid from gaining heat in the case ofliquefied natural gas (LNG) transport and to prevent heat loss in thecase of oil transport. Centralizers made of mechanically strongmaterials (e.g., steel) are used and further insulated with aerogelmaterial to reduce heat conduction through the centralizers. Aerogelmaterial is also inserted in the annular space between the pipes andbetween any two centralizers in the axial direction. If aerogel blanketsare used, the blankets are staggered on top of each other around theedges to limit heat loses. After positioning the blankets, restrictionmeans are used to make sure the blankets do not move out of placeeasily. Such an embodiment can be practiced either at normal externalpressures or at high external pressures such as under the sea systems.The present invention provides ways to transport natural gas as a liquidwhich is otherwise transported as a gas in pipelines. Liquefied naturalgas (LNG) is transported at low temperatures like −250 to 260° F. Evensmall perturbations in the temperature can cause undesirable changes inpressure that have to be taken in to account during the flow pipedesign. The present invention provides flexibility in such designs dueto the effectiveness of the insulation system.

Additional Applications of the Insulation System

The new lightweight, compact, super-insulation system is developed fordeep- and ultra-deep-underwater-structure applications among others. Theinsulation system allows design advantages such as achieving highlyeffective insulation and the possibility of making the spacer contiguousover a long distance. The continuous-spacer design makes the systemquite amenable to the introduction of phase change material or a heatpipe system to increase the effective heat capacity of the system andthereby extend the duration of non-flow mode for the well duringmaintenance or during an extended shut down due to weather, maintenanceor accidents. Two related examples are given below.

Use of a Phase-Change Material (PCM):

A phase-change material (PCM) can be introduced in the space betweenspacers or even inside spacer tubes if the spacer tubes are not used asa heat pipe system. During shut-down periods, the heat stored in the PCMwill be slowly released to the hydrocarbon contained inside the flowline while the insulation covering the PCM will keep the heat insidesubstantially shielded from the cold sea water. By providing excellentthermal insulation between the cold seawater and the PCM, the PCM canmaintain the hydrocarbons above the desired temperature for a longerperiod of time than can a pipe in pipe system with inferior insulationvalue. One example of a suitable PCM is wax, particularlypetroleum/paraffin wax, which can melt and resolidify as the temperatureof the hydrocarbons rises and falls. For example, some oil compositionscan be transported at 160-180° F., where waxes have typical meltingpoints. However, by manipulating the wax composition, the PCM can becreated to have a varied melting point depending on the transporttemperature. The PCM would transfer heat back into the flowinghydrocarbons as the PCM solidifies from a melt.

Geothermal Heat Pipe to Keep the Hydrocarbons Warm During Operation andShutdown:

If the helical spacer coils are equipped with appropriate passages (suchas a fine mesh layer functioning as wicks) for a liquid phase and vaporcore at the center, contiguously connected over a required longdistance, and filled with an appropriate heat-pipe fluid (examplesprovided below), a heat pipe system can be created that utilizes thegeothermal energy from below the sea bottom to keep the hydrocarbons inthe pipe in pipe and other subsea systems above the precipitationtemperature for an extended period during normal operation or evenduring shutdown periods for maintenance or storm. Such an apparatus isillustrated in FIG. 6, where coiled heat pipes 20 fill the gap betweenturns of a coiled spacer 10 on a flow line extending from the oceanfloor, at bottom, through the water, above. An appropriate heat-pipefluid is a fluid that changes its state from liquid to vapor or viceversa within the temperature and pressure limits of the system inoperation. Examples include water, alcohol, glycol, sodium, etc. Asshown in FIG. 7, separate heat pipe tubes 20 run between turns of acoiled spacer 10 and are thermally insulated by aerogel insulationlayers 22.

The heat-pipe fluid moves according to the following path within theconduit. The liquid from the cold side of the conduit is transported bythe surface tension of the liquid acting on the fine wick layer on theinner perimeter of the tube to the hot side where the liquid boils awayand collects into the vapor channel (most prevalently in the coreregion). The vapor pressure generated by boiling drives the vapor towardthe cold region. Once the vapor reaches the cold region, it condenses toliquid and gets into the wick to be sent back to the hot spot by surfacetension induced pumping within the wick layer. The heat pipe system willobviate the need for the electrical heating or other heating methodsthat are very expensive to install, maintain and thus far less desirablethan the heat pipe system, described above.

In describing embodiments of the invention, specific terminology is usedfor the sake of clarity. For purposes of description, each specific termis intended to at least include all technical and functional equivalentsthat operate in a similar manner to accomplish a similar purpose.Additionally, in some instances where a particular embodiment of theinvention includes a plurality of system elements or method steps, thoseelements or steps may be replaced with a single element or step;likewise, a single element or step may be replaced with a plurality ofelements or steps that serve the same purpose. Moreover, while thisinvention has been shown and described with references to particularembodiments thereof, those skilled in the art will understand thatvarious other changes in form and details may be made therein withoutdeparting from the scope of the invention.

1. A method for underwater hydrocarbon transport, the method comprising:A) providing a pipe-in-pipe apparatus comprising: 1) a carrier pipe; 2)a flow line mounted within the carrier pipe; and 3) at least one spacerwithin the carrier pipe, the spacer providing structural support andmaintaining the relative positions of the carrier pipe and the flowline, the spacer enabling the carrier pipe to withstand greater externalpressures without structural failure; B) placing the pipe-in-pipeapparatus underwater at a depth at which the water pressure exceeds theradial collapse strength of the carrier pipe absent the structuralsupport provided by the spacer, where the carrier pipe undergoes acontrolled collapse into catenary-like surfaces between the spacers. 2.The method of claim 1, further comprising the step of flowing ahydrocarbon through the carrier pipe.
 3. The method of claim 2, whereinthe hydrocarbon is a natural gas or crude oil.
 4. The method of claim 1,wherein the spacer serves to partially balance an inward radial forcefrom the seawater pressing against the carrier pipe transmitted from thecarrier pipe through the spacer and transmitted to the flow line withthe outward radial force of a pressurized hydrocarbon inside the flowline, thereby enabling a reduction in wall thickness and diameter of theflow line and carrier pipe for the same level of thermal insulation. 5.The method of claim 1, wherein the spacer includes an aerogel.
 6. Themethod of claim 5, wherein the aerogel is selected from the groupconsisting of precompressed silica aerogels, cellulose aerogels andcombinations thereof.
 7. The method of claim 1, wherein the pipe-in-pipeapparatus further comprises an insulation system positioned between thecarrier pipe and the flow line.
 8. The method of claim 7, wherein theinsulation system includes an aerogel.
 9. The method of claim 7, whereinthe insulation system has a thermal conductivity less than or equal to50 mW/m*K.
 10. The method of claim 7, wherein the pipe-in-pipe apparatusfurther comprises a thermal insulation strip positioned between thespacer and the flow line.
 11. The method of claim 10, wherein theinsulation strip includes an aerogel.
 12. The method of claim 1, whereinthe wall of the carrier pipe partially collapses below a design depthunderwater to form catenary-like surfaces between spacers.
 13. Themethod of claim 12, wherein the primary failure mode of the carrier pipeis tensile failure of the catenary-like surfaces as the depth increases.14. The method of claim 1, wherein the pipe-in-pipe apparatus furthercomprises a weld strip welded to the top of the spacer and to the innerside of the carrier pipe so as to increase the weld integrity and tospread the pressure load transmitted from the carrier pipe to thespacer.
 15. A pipe-in-pipe apparatus for underwater applications, theapparatus comprising: a carrier pipe; a flow line mounted within thecarrier pipe; and at least one spacer within the carrier pipe, thespacer radially separating the carrier pipe from the flow line andproviding sufficient structural support, and the carrier pipe beingsufficiently thin, such that the carrier pipe will form a catenarybetween the spacers under a sufficiently large external compressiveloading, and such that the carrier pipe will fail as a consequence oftensile stress in a catenary region of the carrier pipe rather than as aconsequence of collapse due to buckling under external pressure.
 16. Thepipe-in-pipe apparatus of claim 15, wherein the carrier pipe, by itself,is incapable of withstanding an external compressive load of 25 MPa dueto hydrostatic pressure of sea water, yet the spacer provides sufficientsupport to enable the carrier pipe to withstand the hydrostaticpressure.
 17. The pipe-in-pipe apparatus of claim 15, wherein the spacerincludes an aerogel strip or an aerogel composite.
 18. The pipe-in-pipeapparatus of claim 15, further comprising an aerogel strip mountedbetween the spacer and the flow line, between the spacer and the carrierpipe, or both.
 19. The pipe-in-pipe apparatus of claim 15, wherein thespacer is in the form of a helical coil about the flow line.
 20. Thepipe-in-pipe apparatus of claim 19, wherein the coil is in the form of asolid rod having a cross section in a shape selected from the groupconsisting of circular, elliptical, triangular, and trapezoidal.
 21. Thepipe-in-pipe apparatus of claim 19, wherein the coil is in the form of atube having a cross section in a shape selected from the groupconsisting of circular, elliptical, triangular, and trapezoidal.
 22. Thepipe-in-pipe apparatus of claim 15, wherein the spacer is in the form ofa tubular helical coil about the flow line, and the helical coil tubecontains a heat-transfer medium or a vacuum for thermal management ofthe pipe in pipe.
 23. The pipe-in-pipe apparatus of claim 22, whereinthe helical coil tube contains a heat-transfer medium selected from thegroup consisting of water, alcohol, glycol, sodium and combinationsthereof.
 24. The pipe-in-pipe apparatus of claim 15, wherein the spaceris in the form of discrete rings spaced at substantially uniformintervals along the axial direction of the flow line.
 25. Thepipe-in-pipe apparatus of claim 24, wherein the rings are made of solidrod or tube having a cross section selected from the group consisting ofcircular, elliptical, triangular, and trapezoidal.
 26. The pipe-in-pipeapparatus of claim 15, wherein the overall heat transfer value from thecarrier pipe to the flow line is at most 5 W/m²-° C.
 27. A method forthermally insulating an externally pressure-loaded structure comprising:providing a protective outer skin that lacks sufficient thickness to, byitself, sustain an operational external pressure load acting upon it;providing an underlying structure contained by the protective outerskin; providing at least one thermally insulating spacer with a spacingor pattern that, while supported by the underlying structure,mechanically supports the thin protective outer skin, whilesubstantially inhibiting heat transfer between the protective outer skinand the underlying structure; providing an insulation system having athermal conductivity less than or equal to 50 mW/m-K substantiallyfilling void volumes formed between the underlying structure and theprotective outer skin; and placing the provided elements in anoperational context where the operational external pressure load acts onthe protective outer skin.
 28. The method of claim 27, wherein thestructure is a flow line of a pipe-in-pipe apparatus and the protectiveouter skin is a carrier pipe having a wall thin enough to collapse butsupported by the spacers to form catenary-like surfaces between spacersunder the operational pressure load.
 29. The method of claim 27, whereinthe structure is an underwater subsea tree of pipelines filled with oneor more hydrocarbons.
 30. The method of claim 27, wherein the structureis an underwater riser.
 31. The method of claim 27, wherein thestructure is a liquefied natural gas tanker.
 32. A method for thermallyinsulating a liquefied hydrocarbon flow line comprising: providing aflow line; providing at least one thermally insulating spacer around theflow line; providing an outer carrier pipe that is coaxially alignedwith and around the flow pipe so as to create an annular space betweenthe pipes, wherein the spacer is in that annular space; providing aninsulation system having a thermal conductivity less than or equal to 20mW/m-K, the insulation system substantially filing void volumes outsidethe spacer in the annular space between the flow line and carrier pipe;and flowing a liquefied hydrocarbon through the flow line.
 33. Themethod of claim 32, wherein the insulation system includes an aerogel.34. The method of claim 33, wherein the aerogel is in the form of one ormore blankets.
 35. The method of claim 33, wherein the aerogel is in theform of particles.
 36. The method of claim 32, wherein the hydrocarbonis liquefied natural gas.
 37. The method of claim 32, wherein the spaceris made of an insulation material.
 38. The method of claim 32, whereinthe spacer is made of aerogel material.
 39. The method of claim 1,further comprising monitoring the temperature of the apparatus at aplurality of locations.
 40. The apparatus of claim 15, furthercomprising temperature sensors positioned to measure temperature at aplurality of locations in the apparatus.
 41. The method of claim 27,further comprising monitoring the temperature of the structure at aplurality of locations.
 42. The method of claim 32, further comprisingmonitoring the temperature of the system at a plurality of locations.